Answer:
They get comfortable with how they do things and lose the urge to push themselves outside of their comfort zones. On the other hand, other teachers never stop pushing themselves to learn new things.
The 2nd and 4th one due to the fact it doesn't give a valid reason but more of an effect of doing so.
I don’t see any examples, however, if you look at your examples gloomy is similar to sad or dreary, and the opposite of cheery or happy, so look for that in your examples
